Electrophysiological testing of the human heart has revolutionized the treatment of conduction disturbances and arrhythmias. Application of newer techniques can be expected to improve our understanding of these abnormalities. We plan to define the strength interval, conduction interval, and strength duration relationships of the premature and non-premature beat in the human atrium and ventricle. Patients with a history of atrial or ventricular tachyarrhythmias and conduction slowing will be compared to control subjects. The atrium will be evaluated by endocardial pacing from the high right atrium and intra-atrial conduction time measured from multiple sites in the right and left atrium. Ventricular studies will be performed by pacing the apex and measuring conduction to the ventricular septum. After several drive beats, a premature beat will be induced. Pulse duration and current of the premature and non-premature beats will be varied independently and the threshold necessary to initiate a premature beat tested throughout diastole. The threshold will be plotted as a function of the interstimulus interval between the last drive and premature stimulus. A similar curve of the conduction time as a function of the interstimulus interval will be made. The protocol will be repeated with an early premature beat substituted for the last driven beat. A strength duration curve for early and late diastoles will be made by plotting the threshold as a function of the pulse duration. A cardioactive drug will be administered and the studies repeated. Patients will be followed prospectively. Epicardial pacing will be similarly performed in adult mongrel dogs to determine the validity of bipolar pacing, the significance of repetitive beats and the effect of drugs. These studies will demonstrate electrophysiologic differences between normal and diseased myocardium that may explain the greater arrhythmogenic potential of premature beats in organic heart disease and may identify patients prone to certain arrhythmias.